Fair Value (Fair Value Option - Fair Value and Principal Balances) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Loans Held For Sale | Residential Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	$ 832	$ 1,010
Aggregate Unpaid Principal Balance - Assets	807	998
Difference - Assets	25	12
Loans Held For Sale | Commercial Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	830	1,400
Aggregate Unpaid Principal Balance - Assets	845	1,421
Difference - Assets	(15)	(21)
Loans Held For Sale | Performing Loans | Residential Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	822	1,000
Aggregate Unpaid Principal Balance - Assets	796	988
Difference - Assets	26	12
Loans Held For Sale | Performing Loans | Commercial Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	828	1,395
Aggregate Unpaid Principal Balance - Assets	842	1,412
Difference - Assets	(14)	(17)
Loans Held For Sale | Accruing loans 90 days or more past due | Residential Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	3	4
Aggregate Unpaid Principal Balance - Assets	3	4
Loans Held For Sale | Nonaccrual Loans | Residential Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	7	6
Aggregate Unpaid Principal Balance - Assets	8	6
Difference - Assets	(1)
Loans Held For Sale | Nonaccrual Loans | Commercial Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	2	5
Aggregate Unpaid Principal Balance - Assets	3	9
Difference - Assets	(1)	(4)
Loans | Residential Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	869	893
Aggregate Unpaid Principal Balance - Assets	1,028	1,063
Difference - Assets	(159)	(170)
Loans | Performing Loans | Residential Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	251	247
Aggregate Unpaid Principal Balance - Assets	280	289
Difference - Assets	(29)	(42)
Loans | Accruing loans 90 days or more past due | Residential Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	421	427
Aggregate Unpaid Principal Balance - Assets	431	428
Difference - Assets	(10)	(1)
Loans | Nonaccrual Loans | Residential Mortgage
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	197	219
Aggregate Unpaid Principal Balance - Assets	317	346
Difference - Assets	(120)	(127)
Other Assets
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Assets	216	293
Aggregate Unpaid Principal Balance - Assets	212	288
Difference - Assets	4	5
Other borrowed funds
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value - Liabilities	84	81
Aggregate Unpaid Principal Balance - Liabilities	85	82
Difference - Liabilities	$ (1)	$ (1)